Magnetic resonance imaging has been developed as an imaging technique adapted to obtain both images of anatomical features of human patients as well as some aspects of the functional activities and characteristics of biological tissue. These images have medical diagnostic value in determining the state of the health of the tissue examined. Unlike the situation with fluoroscopic imaging, a patient undergoing magnetic resonance imaging procedure may remain in the active imaging system for a significant amount of time, e.g. a half-hour or more, without suffering any adverse effects.
In a magnetic resonance imaging process, a patient is typically aligned to place the portion of the patient's anatomy to be examined in the imaging volume of the magnetic resonance imaging apparatus. Such a magnetic resonance imaging apparatus typically comprises a primary electromagnet for supplying a constant magnetic field (B0) which, by convention, is along the z-axis and is substantially homogeneous over the imaging volume and secondary electromagnets that can provide linear magnetic field gradients along each of three principal Cartesian axes in space (generally x, y, and z, or x1, x2 and x3, respectively). The magnetic resonance imaging apparatus also comprises one or more radio frequency coils that provide excitation and detection of the magnetic resonance imaging induced signals in the patient's body.
The gradient fields are switched ON and OFF at different rates depending on the magnetic resonance imaging scan sequence used. In some cases, this may result in a changing magnetic field on the order of dB/dt=50 T/s. The frequency that a gradient field may be turned ON can be between 200 Hz to about 300 kHz.
For a single loop with a fixed area, Lenz's law can be stated as:EMF=−A·dB/dt
where A is the area vector, B is the magnetic field vector, and “·” is the vector scalar product. This equation indicates that an electro-motive-force (EMF) is developed in any loop that encircles a changing magnetic field.
In a magnetic resonance imaging system, there is applied to the biological sample (patient) a switched gradient field in all 3 coordinate directions (x-, y-, z- directions). If the patient has an implanted heart pacemaker (or other implanted devices having conductive components) the switched gradient magnetic fields (an alternating magnetic field) may cause erroneous signals to be induced/generated in a sensing lead or device or circuit; damage to electronics; and/or harmful stimulation of tissue, e.g. heart muscle, nerves, etc.
As noted above, the use of the magnetic resonance imaging process with patients who have implanted medical assist devices; such as cardiac assist devices or implanted insulin pumps; often presents problems. As is known to those skilled in the art, implantable devices (such as implantable pulse generators and cardioverter/defibrillator/pacemakers) are sensitive to a variety of forms of electromagnetic interference because these enumerated devices include sensing and logic systems that respond to low-level electrical signals emanating from the monitored tissue region of the patient. Since the sensing systems and conductive elements of these implantable devices are responsive to changes in local electromagnetic fields, the implanted devices are vulnerable to external sources of severe electromagnetic noise, and in particular, to electromagnetic fields emitted during the magnetic resonance imaging procedure. Thus, patients with implantable devices are generally advised not to undergo magnetic resonance imaging procedures.
To more appreciate the problem, the use of implantable cardiac assist devices during a magnetic resonance imaging process will be briefly discussed.
The human heart may suffer from two classes of rhythmic disorders or arrhythmias: bradycardia and tachyarrhythmia. Bradycardia occurs when the heart beats too slowly, and may be treated by a common implantable pacemaker delivering low voltage (about 3 V) pacing pulses.
The common implantable pacemaker is usually contained within a hermetically sealed enclosure, in order to protect the operational components of the device from the harsh environment of the body, as well as to protect the body from the device.
The common implantable pacemaker operates in conjunction with one or more electrically conductive leads, adapted to conduct electrical stimulating pulses to sites within the patient's heart, and to communicate sensed signals from those sites back to the implanted device.
Furthermore, the common implantable pacemaker typically has a metal case and a connector block mounted to the metal case that includes receptacles for leads which may be used for electrical stimulation or which may be used for sensing of physiological signals. The battery and the circuitry associated with the common implantable pacemaker are hermetically sealed within the case. Electrical interfaces are employed to connect the leads outside the metal case with the medical device circuitry and the battery inside the metal case.
Electrical interfaces serve the purpose of providing an electrical circuit path extending from the interior of a hermetically sealed metal case to an external point outside the case while maintaining the hermetic seal of the case. A conductive path is provided through the interface by a conductive pin that is electrically insulated from the case itself.
Such interfaces typically include a ferrule that permits attachment of the interface to the case, the conductive pin, and a hermetic glass or ceramic seal that supports the pin within the ferrule and isolates the pin from the metal case.
A common implantable pacemaker can, under some circumstances, be susceptible to electrical interference such that the desired functionality of the pacemaker is impaired. For example, common implantable pacemaker requires protection against electrical interference from electromagnetic interference (EMI), defibrillation pulses, electrostatic discharge, or other generally large voltages or currents generated by other devices external to the medical device. As noted above, more recently, it has become crucial that cardiac assist systems be protected from magnetic-resonance imaging sources.
Such electrical interference can damage the circuitry of the cardiac assist systems or cause interference in the proper operation or functionality of the cardiac assist systems. For example, damage may occur due to high voltages or excessive currents introduced into the cardiac assist system.
Moreover, problems are realized when the placement of the implant is next to particular organs. For example, when a pacemaker is placed in the upper chest and the lead tip is placed into the heart, a loop (an electrical loop) is created. A changing magnetic field (the switched gradient field) over the area of the loop (through the area of the loop) will cause an induced voltage (and current) across the heart. This induced voltage (current) can stimulate the heart inappropriately and can cause heart damage or death.
It is further noted that uncoated wire forms used by in-vivo devices, such as pacing leads, do not have significantly less DC resistance than coated wire forms because it appears that the current path is the same for the uncoated and coated wires. In other words, an apparent oxidation layer on the uncoated wire results in essentially a resistive coating over the “uncoated” wire form. Thus, the current does not flow through adjacent coil loop contact points but instead follows the curvature of the wire, just as in the case of the coated wire.
At a magnetic resonance imaging scanner's frequency, approximately 63.85 MHz, both uncoated and coated wire forms have the characteristics of capacitors, not inductors. It is apparent that parasitic capacitance is formed between adjacent loops in the wire and is the dominate characteristic of the wire at approximately 63.85 MHz. This parasitic capacitance enables electrical current to flow into and out of the wire form due to several mechanisms, including the oscillating electrical field set up in the body by the magnetic resonance imaging unit. In a pacing lead, this condition creates a high current density at the exposed electrode located at the distal end of the lead.
Therefore, it is desirable to provide wire forms which shift the resonance frequency of the wire form towards approximately 63.85 MHz, the resonance frequency of a magnetic resonance imaging scanner, thereby changing the wire form's characteristics to that of an inductor, which has the property of adding impedance to the flow of AC current induced by the magnetic resonance imaging unit.